LibWeb: Move DOM classes into the Web::DOM namespace

LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.

Libraries/LibWeb/DOM/ElementFactory.cpp

@@ -51,7 +51,7 @@
 #include <LibWeb/SVG/SVGSVGElement.h>
 #include <LibWeb/SVG/TagNames.h>
 
-namespace Web {
+namespace Web::DOM {
 
 NonnullRefPtr<Element> create_element(Document& document, const FlyString& tag_name)
 {